COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN 379 



normal salts, soluble in water, but having an alkaline reaction and in 

 many cases acting themselves as alkalis. K) The acid salts of these alkalis,. 

 NaHCO 3 and KHCO 3 , have a neutral reaction on litmus, although they, 

 like acids, contain hydrogen, which may be exchanged for metals. The 

 acid salts of such acids as, for instance, of sulphuric acid, NaHSO 4 

 have a clearly-defined acid reaction, and therefore carbonic acid is un- 

 able to saturate the powerful basic properties of such alkalis as potash or 

 soda. Carbonic acid does not even combine at all with feeble bases, such 

 as alumina, A1 2 O 3 , and therefore if a strong solution of sodium carbo- 

 nate, Xa.,CO 3 , be added to a strong solution of aluminium sulphate 

 A1.>(SO 4 ) 3 , then (according to double saline decompositions) aluminium 

 carbonate, A1 2 (CO 3 ) 3 , ought to be formed, but the carbonic acid separates, 

 as this salt splits up in the presence of water into aluminium hydrox- 

 ide and carbonic anhydride: A1 2 (CO 3 ) 3 + 3H 2 O = A1 2 (OH) 6 + 3CO 2 . 

 Therefore feeble bases are unable to retain carbonic acid even at 

 ordinary temperatures. For this reason in the case of bases of medium 

 energy, although they form carbonates, these are comparatively easily 

 decomposed by heating, as is shown by the decomposition of copper 

 carbonate, CuCO 3 (see Introduction), and even calcium carbonate, 

 CaCO 3 . Only the normal (but not the acid) salts of such powerful bases, 

 as potassium or sodium are capable of standing a red heat without 

 decomposition. But the acid salts, for instance NaHCO 3 , decompose 

 even on heating their solutions (2NaHC0 3 = Na 2 CO 3 + H 2 O + CO 2 ), 

 evolving carbonic anhydride. The amount of heat given out by the 

 combination of carbonic acid with bases also shows its feeble acid 

 properties, because it is considerably less than with energetic acids. 

 Thus if a weak solution of forty grams of sodium hydroxide be satu- 

 rated (until the formation of a normal salt) with sulphuric or nitric 

 acid or another powerful acid, from thirteen to fifteen thousand 

 calories are given out, but with carbonic acid only about ten thousand 

 calories. 16 The majority of carbonates are insoluble in water, and 



15 Thus, for instance, in the washing of fabrics the caustic alkalis, such as sodium 

 hydroxide, in weak solutions, act in removing the fatty matter just in the same way as 

 carbonate solutions ; for instance, a solution of soda crystals, Na. 2 CO 3 . Soap, acts in the 

 same way, being composed of feeble acids, either fatty or resinous, combined with alkali. 

 On this account all such substances are applied to manufacturing purposes, and used 

 equally well in practice for bleaching and washing fabrics. Soda crystals or soap are 

 preferred to caustic alkali, because an excess of the latter may have a destructive effect 

 on the fabrics. It may be supposed that in aqueous solutions with soap or with soda, 

 crystals part of the base will form caustic alkali ; that is to say, the water will compete 

 with the weak acids, and the alkali will be distributed between them and the water. 



10 Although carbonic acid is reckoned among the feeble acids, yet there are evi- 

 dently many others still feebler for instance, hydrogen sulphide, prussic acid, hypo- 

 chlorous acid, many organic acids, ifcc. Bases like alumina, or such feeble acids as silica,, 

 when in combination with alkalis are decomposed in aqueous solutions by carbonic acid> 



